Candidate cloning – executive summary

If a second candidate A, almost identical to B in the eyes of all voters,
enters the race (or withdraws) that should neither hurt nor help
B's chances of being elected (aside
from replacement by a clone).
More precisely: "clones" in rank-order-ballot systems all are ranked adjacent by every voter.
In ratings-type systems they are all rated within ±ε by every voter in the
limit ε→0.

But Plurality, Borda,
and Copeland voting all hugely-fail this test.
That makes election manipulation easy (plus often
more cost-effective than "running an honest race").